According to John Schlitt, Petra's lead singer, Petra is a ministry first and a band second. Petra seeks God's will, and sometimes He directs them to make changes in the band's membership. Recently, Bob Hartman rejoined the band as guitarist and Paul Simmons was hired as their new drummer.
Previous Band Members
Below is a listing of the various people who have filled different positions for Petra over the years. Keep in mind that I am a little sketchy on the early days of Petra, so not all the dates are perfect.
Bob Hartman 1972-1995, 2003-Now
John Slick 1981-1983
Bill Glover 1972-1978?
John DeGroff 1972-1978?
Bob Hartman & Greg Hough 1972-1977
These are pictures of Petra from sometime in the early or middle 70s. I found them on the Jesus Music webpage.